The invention relates to improvements for increasing the pumping efficiency of vertical cantilever pumps of the type adapted to handle highly corrosive and/or abrasive liquids.
A typical prior art cantilever pump construction, such as that disclosed in U.S. Pat. No. 2,355,472, includes a vertically extending pump drive shaft, which serves to mount an impeller adjacent its lower end and is arranged to extend downwardly through a pedestal, which in turn serves to mount an impeller enclosing pump head casing or housing. The corrosive and/or abrasive nature of the liquid being pumped requires the provision of a relatively loose fit between the pump shaft and the pump head casing with the result that a portion of the pumped liquid is free to escape from the casing upwardly along the pump shaft. Normally, the pedestal is of an open construction in order to permit the escaping liquid to be immediately returned to the main body of the liquid being pumped before it can rise to a height at which it will escape from the liquid receiving tank and/or damage the pump drive motor or its bearings. Thus, a decided drawback of the type of pump construction disclosed in U.S. Pat. No. 2,355,472 is that the "loss" of pumped liquid results in a substantial reduction in the efficiency of the pump.
In an effort to minimize the internal leakage under operating conditions and thus increase the potential output or discharge of the pump for a given capacity pump motor, it has been proposed to form a pedestal of substantially closed construction such that the escaping liquid establishes a column of liquid opposing further escape. However, as a practical matter, during a normal pumping operation, the "safe height" of this liquid column, i.e. the height below that at which liquid might escape from the holding tank is insufficient to arrest the escape of liquid from the pump chamber, and thus the side wall of the pedestal is necessarily formed with openings to permit continuous bleeding or flow of liquid from the column back into the liquid bath. Moreover, from time to time a back pressure may be encountered in the pump discharge conduit, which is substantially in excess of normal pump discharge pressure, as for instance due to closing of an outlet valve, while the pump is in operation. This increase in back pressure will tend to produce a substantial increase in the normal height of the column of escaping liquid, thereby to reduce the value of the so-called safe height of the column and as a result limit the efficiency of the pump.
U.S. Pat. No. 2,622,537 discloses a cantiliver pump construction, which represents a compromise between the fully open and substantially closed pedestal constructions described above. Specifically, in this pump construction the pedestal is substantially closed in order to provide a column of liquid within the pedestal for opposing escape of pumped liquid, while at the same time the interior of the pedestal is placed in communication with the inlet of the pump in order to continuously withdraw escaping fluid from the pedestal.